Thiol-based Redox Control of Tetrapyrrole Metabolism: Posttranslational Control of Tetrapyrrole Biosynthesis Enzymes by NADPH-Dependent Thioredoxin Reductase C (NTRC) and Thioredoxins

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Plants effectively coordinate the photosynthesis dependent production of energy and reducing equivalents with the activity of metabolic pathways. As the redox state of chloroplasts changes in dependency to photosynthesis activities, it is proposed that the dark suppression and light activation of 5-aminolevulinic acid (ALA) synthesis is coupled with redox control via modification of thiol groups of contributing enzymes. My group is interested in the elucidation of the posttranslational control of tetrapyrrole biosynthesis and we identified several enzymes of this pathway to be targets of the reversible thiol-disulfide redox regulation by the cooperative action of the redox regulators NADPH-dependent thioredoxin reductase C (NTRC) and thioredoxin (TRX). This posttranslational control mechanism balances tetrapyrrole biosynthesis in response to environmental and endogenous cues and prevents accumulation of photoreactive tetrapyrrole metabolites. In continuation we address in the follow-up proposal the two main objectives: (i) Exploration of the working hypothesis that the dark-dependent inactivation of glutamyl-tRNA reductase (GluTR) by the negative regulator FLU (FLOURESCENT) is based on a redox control mechanism and (ii) investigation of the thiol-based redox switches of two additional enzymes in the pathway, ALA dehydratase (ALAD) and protoporphyrinogen oxidase (PPOX) and elucidation of the physiological impact of the redox control on the modified activity and stability of these two target enzymes. It is expected that elucidation of these details of thiol-disulfide exchange reactions on enzymes of tetrapyrrole biosynthesis will improve understanding on redox-controlled tetrapyrrole biosynthesis, in particular the essential light-dark activation/inactivation of ALA synthesis by GluTR in higher plants.